Esterification as a diagnostic tool to predict proton conductivity affected by impurities on Nafion components for proton exchange membrane fuel cells

Quantitative data of the effect of contaminants on individual components of a PEMFC is limited and difficult to acquire, especially for the ionomer in the catalyst layer. In this paper, we propose the use of an acid-catalysed reaction (esterification) as a method to quantitatively investigate the effect of contaminants on proton availability and conductivity of Nafion components, since proton sites in Nafion are also active as Brønsted acid sites for catalysis. It was found that at typical fuel cell conditions, ammonia adsorption decreased both conductivity and esterification activity of Nafion in a uniform manner. Because of the linear relationship between the number of proton/acid sites and both the conductivity and the esterification activity, a correlation between the two could be developed taking into account differences in the effect of humidity on the conductivity/activity of the poisoned Nafion. The methodology and correlation developed were also shown to predict accurately the effect of another impurity species (Na⁺) on Nafion conductivity. The results demonstrate the application of esterification as a means to quantify the number of proton sites poisoned by adsorbing impurities, permitting the prediction of Nafion conductivity. This method would be applicable to both the membrane and ionomer in the catalyst layer.     

Effect of cations (Na, Ca, Fe) on the conductivity of a Nafion membrane

It is known that trace amounts of cations have a detrimental effect on the liquid-phase conductivity of perfluorosulfonated membranes at room temperature. However, the conditions used were very different from typical fuel cell conditions. Recent research has shown the impact of conductivity measurement conditions on NH₄⁺ contaminated membranes. In this study, the impact of nonproton-containing cations (Mⁿ⁺ = Na⁺, Ca²⁺, and Fe³⁺) on Nafion membrane (N-211) conductivity was investigated both in deionized (DI) water at room temperature (∼25 °C) and in the gas phase at 80 °C under conditions similar to in a PEMFC. These conductivities were compared with those of Nafion membranes contaminated with NH₄⁺ ions. Under the same conditions, the conductivity of a metal cationic-contaminated membrane having the same proton composition (yH⁺m) was similar, but slightly lower than that of an NH₄⁺-contaminated membrane. The conductivity in the purely H⁺-form of N-211 was more than 12 times greater than the Mⁿ⁺-form form at 25 °C in DI water. At 80 °C, the gas-phase conductivity was 6 times and 125 times greater at 100%RH and 30%RH, respectively. The quantitative results for conductivity and activation energy of contaminated membranes under typical fuel cell conditions are reported here for the first time.     

Effect of H2O2 on Nafion properties and conductivity at fuel cell conditions

During PEM fuel cell operation, formation of H₂O₂ and material corrosion occurs, generating trace amounts of metal cations (i.e., Fe²⁺, Pt²⁺) and subsequently initiating the deterioration of cell components and, in particular, PFSA membranes (e.g., Nafion). However, most previous studies of this have been performed using conditions not relevant to fuel cell environments, and very few investigations have studied the effect of Nafion decomposition on conductivity, one of the most crucial factors governing PEMFC performance. In this study, a quantitative examination of properties and conductivities of degraded Nafion membranes at conditions relevant to fuel cell environments (30-100%RH and 80 °C) was performed. Nafion membranes were pre-ion-exchanged with small amounts of Fe²⁺ ions prior to H₂O₂ exposure. The degradation degree (defined as loss of ion-exchange capacity, weight, and fluoride content), water uptake, and conductivity of H₂O₂-exposed membranes were found to strongly depend on Fe content and H₂O₂ treatment time. SEM cross-sections showed that the degradation initially took place in the center of the membrane, while FTIR analysis revealed that Nafion degradation preferentially proceeds at the sulfonic end group and at the ether linkage located in the pendant side chain and that the H-bond of water is weakened after prolonged H₂O₂ exposure.     

Investigation of the effects of SPEEK and its clay composite membranes on the performance of Direct Borohydride Fuel Cell

In this study, composite cation exchange membranes (CEM) were developed. With the experience from widely studied proton exchange membrane fuel cells (PEMFC), sulfonated polyether ether ketone (SPEEK) was prepared to be a more effective and cheaper ionomer alternative to the industry standard Nafion ®. SPEEK polymer membrane can reach sufficient ionic conductivities but have some mechanical and chemical stability problems (at a high degree of sulfonations (DS)). Therefore, in order to optimize the membrane, composite mixing with a well-known organic/inorganic clays called Cloisite® 15A, Cloisite ® 30B and MMT were used. Test cells for both single-cell and conductivity were designed and constructed. The ionic conductivity cell was different than the ones used in most studies, measuring conductivity in-plane with 4 probes using EIS. The membranes were characterized for their proton conductivity with electrochemical impedance spectroscopy (EIS), for DS with H NMR, water uptake, and fuel cell performance tests. First results showed that the acidic sulfonic groups of SPEEK interacted with organic/inorganic clays and as a result of partial barrier the ionic conductivity was decreased but power densities were increased. SPEEK-Cloisite® 30B composite membrane has given 40 mW/cm² power density value which is higher than pure SPEEK membrane (35 mW/cm²). The proton conductivities of the final composite membranes were close to bare SPEEK membranes which are 0,065 and 0,075 S/cm for SPEEK-Cloisite ® 30B and pristine SPEEK, respectively.     

Investigation of physical properties and cell performance of Nafion/TiO2 nanocomposite membranes for high temperature PEM fuel cells

Synthesis and characterization of Nafion/TiO₂ membranes for proton exchange membrane fuel cell (PEMFC) operating at high temperatures were investigated in this study. Nafion/TiO₂ nanocomposite membranes have been prepared by in-situ sol–gel and casting methods. In the sol–gel method, preformed Nafion membranes were soaked in tetrabutylortotitanate (TBT) and methanol solution. In order to compare synthesis methods, a Nafion/TiO₂ composite membrane was fabricated with 3 wt.% of TiO₂ particles by the solution casting method. The structures of membranes were investigated by Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), and Energy Dispersive X-Ray Analysis (EDXA). Also, water uptake and proton conductivity of modified membranes were measured. Furthermore, the membranes were tested in a real PEMFC. X-Ray spectra of the composite membranes indicate the presence of TiO₂ in the modified membranes. In case of the same doping level, sol–gel method produces more uniform distribution of Ti particles in Nafion/TiO₂ composite membrane than the ones produced by casting method. Water uptake of Nafion/TiO₂ membrane with 3 wt.% of doping level was found to be 51% higher than that of the pure Nafion membrane. EIS measurements showed that the conductivity of modified membranes decreases with increasing the amount of doped TiO₂. Finally, the membrane electrode assembly (MEA) prepared from Nafion/Titania nanocomposite membrane shows the highest PEMFC performance in terms of voltage vs. current density (V–I) at high temperature (110 °C) which is the main goal of this study.     

Improvement of PEMFC performance with Nafion/inorganic nanocomposite membrane electrode assembly prepared by ultrasonic coating technique

Membrane electrode assemblies with Nafion/nanosize titanium silicon dioxide (TiSiO₄) composite membranes were manufactured with a novel ultrasonic-spray technique and tested in proton exchange membrane fuel cell (PEMFC). Nafion/TiO₂ and Nafion/SiO₂ nanocomposite membranes were also fabricated by the same technique and their characteristics and performances in PEMFC were compared with Nafion/TiSiO₄ mixed oxide membrane. The composite membranes have been characterized by thermogravimetric analysis, scanning electron microscopy, X-ray diffraction, water uptake, and proton conductivity. The composite membranes gained good thermal resistance with insertion of inorganic oxides. Uniform and homogeneous distribution of inorganic oxides enhanced crystalline character of these membranes. Gas diffusion electrodes (GDE) were fabricated by Ultrasonic Coating Technique. Catalyst loading was 0.4 mg Pt/cm² for both anode and cathode sides. Fuel cell performances of Nafion/TiSiO₄ composite membrane were better than that of other membranes. The power density obtained at 0.5 V at 75 °C was 0.456 W cm⁻², 0.547 W cm⁻², 0.477 W cm⁻² and 0.803 W cm⁻² for Nafion, Nafion/TiO₂, Nafion/SiO₂, and Nafion/TiSiO₄ composite membranes, respectively.     

Role of UV irradiated Nafion in power enhancement of hydrogen fuel cells

The influence of optimum UV ray exposure of pristine Nafion polymer membranes on the improvement of proton conductivity and hydrogen fuel cell performance has been examined. Nafion membranes with thickness 183  μm (117), 90  μm (1035), 50  μm (212) and 25  μm (211) were irradiated with ultraviolet rays with doses in the range 0–250 mJ cm^?2 and their proton conductivities have been measured with standard method. The Nafion membranes have also been studied by measuring their water uptake, swelling-ratios and porosity using standard procedures. Hydrogen fuel cells with dual serpentine microchannels with active area 1.9 x 1.6 cm^-2 were assembled with anode gas diffusion layer, Nafion membrane, cathode gas diffusion layer and other components. An external humidifier was used to humidify hydrogen for the fuel cell. The experimental Results have shown an increase in the value of Nafion proton conductivity with an optimum UV irradiation which depends on the thickness of Nafion membrane: the optimum doses for peak proton conductivity were 196mJ/cm^?2 for Nafion 117, 190mJ/cm^?2for Nafion 1035, 180mJ/cm^?2 for Nafion 212 and 160mJ/cm^?2 for Nafion 211. This enhancement of proton conductivity is because of the optimal photo-crosslinking of –SO₃H groups in Nafion. This causes optimum pore-size in Nafion thereby facilitating increased proton-hopping between –SO₃H sites in Nafion. Hydrogen fuel cells were developed with pristine as well as with optimal UV irradiated Nafion with thicknesses of 90 and 50  μm. The polarization plots obtained for these devices showed an increase in power densities approximately by a factor of 1.8–2.0 for devices with optimally UV irradiated Nafion. These results indicate that optimal UV irradiation of Nafion is an excellent technique for enhancing power output of hydrogen fuel cells.     

Prediction of the effective conductivity of Nafion in the catalyst layer of a proton exchange membrane fuel cell

In a previous study, a simple acid catalyzed reaction (esterification) was found to predict excellently conductivity of a membrane contaminated with NH₄⁺ or Na⁺. Since measurement of the conductivity of Nafion in a catalyst layer is problematic, being able to predict this conductivity for various formulations and fuel cell conditions would be advantageous. In this study, the same methodology as before was used to examine the proton availabilities of supported Nafion (Nafion on carbon and on Pt/C), as exists in the catalyst layer used in a PEMFC, during impurity exposure (e.g., NH₃) as a means for prediction of its conductivity. It was found that the effect of NH₃ exposure on the proton composition (yH+) of supported Nafion was similar to that of N-211 under the same conditions. Determined values of yH+ were then used to estimate the effective conductivity of an ammonium-poisoned cathode layer using the correlation developed and the agglomerate model. The predicted conductivities were matched with the results available in the literature. This technique would be useful for the optimization of catalyst design and for fuel cell simulation, since it provides many benefits over conventional performance test procedures.     

Self-assembly of highly charged polyelectrolyte complexes with superior proton conductivity and methanol barrier properties for fuel cells

The paper is concerned with the formation of Layer-by-Layer (LbL) self-assembly of highly charged polyvinyl sulfate potassium salt (PVS) and polyallylamine hydrochloride (PAH) on Nafion membrane to obtain the multilayered composite membranes with both high proton conductivity and methanol blocking properties. Also, the influences of the salt addition to the polyelectrolyte solutions on membrane selectivity (proton conductivity/methanol permeability) are discussed in terms of controlled layer thickness and charge density.The deposition of the self-assembly of PAH/PVS is confirmed by SEM analysis and it is observed that the polyelectrolyte layers growth on each side of Nafion membrane regularly. (PAH/PVS)₁₀-Na⁺ and (PAH/PVS)₁₀-H⁺ with 1.0 M NaCl provide 55.1 and 43.0% reduction in lower methanol permittivity in comparison to pristine Nafion, respectively, while the proton conductivities are 12.4 and 78.3 mS cm⁻¹. Promisingly, it is found that the membrane selectivity values (Φ) of all multilayered composite membranes in H⁺ form are much higher than those of Na⁺ form and perfluorosulfonated ionomers reported in the literature. These encouraging results indicate that composite membranes having both superior proton conductivity and improved methanol barrier properties can be prepared from highly charged polyelectrolytes including salt for fuel cell applications.     

Preparation and characterization of phosphotungstic acid-derived salt/Nafion nanocomposite membranes for proton exchange membrane fuel cells

Nafion/Cs_2.5H_0.5PW₁₂O₄₀ nanocomposite membranes are prepared and characterized as alternate materials for PEMFC operation at high temperature/low humidity. The Cs_2.5H_0.5PW₁₂O₄₀ solid acid particles (hereafter CsPWA) have the high surface area, the high hygroscopic property and the ability to generate proton in the presence of water molecules. The results of prepared membranes at three levels (0, 10 and 15%) indicate that the CsPWA particles have influence on the water content, ion exchange capacity, thermal properties (TGA and DSC), proton conductivity and PEM fuel cell performance. Particles agglomeration and Nafion active sites (sulfonic groups) covering are seen in the nanocomposite membranes. The conductivity of nanocomposite membranes at high temperatures (110 and 120 °C) is higher than plain Nafion and may be related to the additional water within the nanocomposite membrane and/or the additional surface functional site provide by CsPWA. The fuel cell responses show that in the fully hydrated state and at the higher current densities, the prepared MEAs with nanocomposite membranes possess better response compared with the plain Nafion. In partially hydrated cell, at both low and high current densities, the superior performance of the MEA prepared by nanocomposite membranes is observed.     

Preparation of Nafion-sulfonated clay nanocomposite membrane for direct menthol fuel cells via a film coating process

Nafion sulfonated clay nanocomposite membranes were successfully produced via a film coating process using a pilot coating machine. For producing the composite membranes, we optimized the solvent ratio of N-methyl-2-pyrrolidinone (NMP) to N,N′-dimethylacetamide (DMAc), the amount of sulfonated montmorillonite (S-MMT) in composite membranes and the overall concentration of composite dispersions. Based on the optimized viscosity and composition, the composite dispersions were coated on a poly(ethylene terephthalate) (PET) substrate film. The distance between a metering roll and a PET film and the ratio of metering roll speed versus coating roll speed of the pilot coating machine were varied to control membrane thickness. The film coated composite membrane exhibited enhanced properties in the swelling behavior against MeOH solution, ion conductivity and MeOH permeability, compared to the cast Nafion composite membrane due to the higher dispersion state of S-MMT in Nafion matrix and the uniform distribution of small-size ion clusters. These properties influenced a cell performance test of a direct methanol fuel cell (DMFC), showing the film coated composite membrane had a higher power density than that of Nafion 115. The power density was also related with the higher selectivity of the composite membrane than Nafion 115.     

Modeling of ion conductivity in Nafion membranes

A theoretical investigation was conducted to describe the ion transport behavior in a Nafion Membrane of proton exchange membrane fuel cells (PEMFC). By analyzing the surface energy configuration of the ionic clusters in a Nafion membrane, an equivalent field intensity, E _e , was introduced to facilitate the analysis of surface resistance against ion conduction in the central region of clusters. An expression was derived for ionic conductivity incorporating the influence of surface resistance. A face-centered cubic (FCC) lattice model for a spatial cluster distribution was used to modify the effect of water content on ionic conductivity in the polymeric matrix, i.e., the regions between clusters. Compared with the available empirical correlations, the new expression showed much better agreement with the available experimental results, which indicates the rationality to consider the structural influence on ion conduction in water-swollen Nafion membranes.     

Nafion/Pd–SiO2 nanofiber composite membranes for direct methanol fuel cell applications

One of the major challenges for direct methanol fuel cells is the problem of methanol crossover. With the aim of solving this problem without adverse effects on the membrane conductivity, Nafion/Palladium–silica nanofiber (N/Pd–SiO₂) composite membranes with various fiber loadings were prepared by a solution casting method. The silica-supported palladium nanofibers had diameters ranging from 100 nm to 200 nm and were synthesized by a facile electro-spinning method. The thermal properties, ionic exchange capacities, water uptake, proton conductivities, methanol permeabilities, chemical structures, and micro-structural morphologies were determined for the prepared membranes. It was found that the transport properties of the membranes were affected by the fiber loading. All of the composite membranes showed higher water uptake and ion exchange capacities compared to commercial Nafion 117 and proved to be thermally stable for use as proton exchange membranes. The composite membranes with optimum fiber content (3 wt%) showed an improved proton conductivity of 0.1292 S cm⁻¹ and a reduced methanol permeability of 8.36 × 10⁻⁷ cm² s⁻¹. In single cell tests, it was observed that, the maximum power density measured with composite membrane is higher than those of commercial Nafion 117.     

Novel multilayer Nafion/SPI/Nafion composite membrane for PEMFCs

A novel multilayer membrane for the proton exchange membrane fuel cell (PEMFC) was developed. Nafion was dispersed uniformly onto both sides of the sulfonated polyimide (SPI) membrane. The Nafion/SPI/Nafion composite membrane was prepared by immersing the SPI into the Nafion-containing casting solution. Through immersing both membranes into the Fenton solution at 80 °C for 0.5 h for an accelerated ex situ test, chromatographic analysis of the water evacuated from the cathode and the anode of the cells and a durability test of a single proton exchange membrane fuel cells, it was proved that the stability of the composite membrane has been greatly improved by adding the Nafion layer compared with the SPI membrane. The fuel cell performance with the SPI and Nafion/SPI/Nafion membranes was similar to the performance with the commercial product Nafion^® NRE-212 membrane at 80 °C.     

Novel ultrathin composite membranes of Nafion/PVA for PEMFCs

The electrospinning approach is an easy and useful method to fabricate porous supports with tailored properties for the preparation of impregnated membranes with enhanced characteristics. Therein, this technique was used to obtain polyvinyl alcohol (PVA) nanofiber mats in which Nafion^® polymer was infiltrated. These Nafion/PVA membranes were characterized in their mechanical properties, proton conductivity and fuel cell performance. Conductivity of the composite membranes was below the showed by pristine Nafion^® due to the non-ionic conducting behaviour of the PVA phase, although the incorporation of the PVA nanofibers strongly reinforced the mechanical properties of the membranes. Measurements carried out in a single cell fed with H₂/Air confirmed the high performance exhibited by a 19 μm thick nanofiber reinforced membrane owing to its low ionic resistance. These reasons make ultrathin (<20 μm) Nafion/PVA composite membranes promising candidates as low cost ion-exchange membranes for fuel cell applications.     

Synthesis and characterization of sulfonated polysulfone membranes for direct methanol fuel cells

Sulfonated polysulfones (SPSf) with different degree of sulfonation (DS) have been synthesized and evaluated as proton exchange membranes in direct methanol fuel cell (DMFC). The membranes have been characterized by ion exchange capacity (IEC), proton conductivity, liquid uptake, and single DMFC polarization measurements. The proton conductivities of the SPSf membranes increase with increasing sulfonation, but are lower than that of Nafion 115. Within the range of sulfonation of 50–70%, the SPSf membranes exhibit better performances in DMFC than Nafion 115 at lower methanol concentrations (1 M) despite lower proton conductivities due to suppressed methanol permeability and crossover. However, the performances of SPSf membranes at higher methanol concentrations (2 M) are inferior to that of Nafion 115 at current densities higher than about 50 mA cm⁻² as the suppression in methanol crossover could not quite compensate for the lower proton conductivities.     

Influence of ammonia on the conductivity of Nafion membranes

The effect of NH₃ and NH₄⁺ poisoning on the conductivity of Nafion membranes was investigated via electrochemical impedance spectroscopy. The conductivities of membranes prepared with different NH₄⁺ compositions were measured in deionized water at room temperature and compared to those at 80 °C in a gas phase for various relative humidities. The liquid-phase conductivity decreased linearly with an increase in the NH₄⁺ composition in the membrane (y_NH4⁺), with that of the NH₄⁺-form having a conductivity 25% that of the H⁺-form. The gas-phase conductivity of the NH₄⁺-form, on the other hand, declined by 66–98% relative to the H⁺-form depending on humidity. The conductivities of fresh membranes in the presence of gas-phase NH₃ at different humidities were also studied. The conductivity decreased with time-on-stream and reached the same conductivity at a given humidity regardless of the NH₃ concentration, but the time to reach steady-state varied with NH₃ concentration. The y_NH4⁺ at steady-state conductivity was equivalent for all the NH₃ concentrations studied. The kinetics of conductivity decrease was slower at higher humidities. The humidity and y_NH4⁺ appear to have a concerted effect on the conductivity. The quantitative conductivity data under practical fuel cell conditions should be useful for future fuel cell modeling.     

Fabrication and evaluation of Nafion nanocomposite membrane based on ZrO2–TiO2 binary nanoparticles as fuel cell MEA

Synthesis and characterization of nanocomposite membranes for proton exchange membrane fuel cell (PEMFC) operating at different temperatures and humidity were investigated in this study. Recast Nafion composite membrane with ZrO₂ and TiO₂ nanoparticles with 75 nm in mean size diameter, prepared for PEM fuel cells. Nafion/TiO₂ composite membranes have been also fabricated by in-situ sol–gel method. However, fine particles of the ZrO₂ were synthesized and Nafion/ZrO₂ composite membrane were produced by blending a 5% (w/w) Nafion-water dispersion with the inorganic compound. All nanocomposite membranes demonstrated higher water retention in comparison with unmodified membranes. Proton conductivity increased with increasing ZrO₂ content while TiO₂ additive (with mean size of 25 nm) enhanced water retention. Subsequently, structures of the membranes were investigated by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) as well as X-Ray Diffraction (XRD). In addition, water uptake and proton conductivity of the modified membranes were also measured. The nanocomposite membrane was tested in a 25 cm² commercial single cell at the temperature range of 80–110 °C and in humidified H₂/O₂ under different relative humidity (RH) conditions. The membrane electrode assembly (MEA) prepared from Nafion/TiO₂, ZrO₂ presented highest PEM fuel cell performance in respect of I–V polarization under condition of 110 °C, 0.6 V and 30% RH and 1 atm.     

Direct methanol fuel cell membranes from Nafion–polybenzimidazole blends

Proton conducting membranes for a direct methanol fuel cell (DMFC) were fabricated from blends of Nafion^® and polybenzimidazole (PBI) by solution casting. Prior to dissolution in the casting solvent, the sulfonic acid groups of the Nafion component of the blend were partially exchanged with sodium ions. The dependence of membrane proton conductivity and methanol permeability on the extent of proton substitution of Nafion during blending and on the PBI content of the final membrane was studied. It was found that membrane selectivity (the ratio of proton conductivity to methanol permeability) was the highest (four times that of Nafion 117) when fully protonated Nafion was used during blending and when the PBI content was 8%. DMFC performance of Nafion–PBI membranes (approximately 60 μm in thickness) was found to be superior to that of Nafion 117 at 1.0 and 5.0 M methanol feeds.     

Nafion/sulfated β-cyclodextrin composite membranes for direct methanol fuel cells

Proton-conducting composite membranes based on H⁺-form sulfated β-cyclodextrin (sb-CD) in a Nafion matrix are prepared via the solution-casting method and their methanol permeabilities, proton conductivities, proton diffusion coefficients and cell performances are measured. The methanol permeabilities of the composite membranes increase very slightly with increases in their sb-CD content. As a result of adding sb-CD with its many sulfonic acid groups into the Nafion matrix, the proton conductivities of the composite membranes increase with increases in their sb-CD content. The methanol permeability and proton conductivity results are used to show that the best selectivity of the membranes is that of the NC5 membrane (‘NCx’ denotes a Nafion/sb-CD composite membrane containing x wt.% sb-CD). The proton diffusion coefficients are measured with ¹H pulsed field gradient nuclear magnetic resonance (PFG-NMR) and found to increase with increase in the sb-CD content in the order NC5 > NC3 > NC1 > NC0. Thus the presence of sb-CD in the Nafion membranes increases the proton diffusion coefficients as well as the proton conductivities, ionic cluster size, water uptakes and the ion-exchange capacities (IECs). A maximum power density of 58 mW cm⁻² is obtained for the NC5 membrane. The combination of these effects should lead to an improvement in the performance of direct methanol fuel cells prepared with Nafion/sb-CD composite membranes.